The measurement of the natural presence of oil on the human facial skin is a problem of current interest among cosmetics manufacturers, since the quantitative assessment of skin oiliness enables the beautician or cosmetics sales clerk to recommend an optimal assortment of cosmetics to each individual customer.
It has been proposed in the prior art to perform the skin oiliness measurement by first contacting a ground glass in the form of a small sampling pellet to the subject's skin, and then detecting a change in the light-scattering properties of the groundglass. Prototype instruments based on this scheme have been constructed, which include a light-emitting diode (LED), and a silicon light detector placed within a small portion of the luminous emission field of the LED. The groundglass is inserted between the LED and the detector so that it intercepts some radiation from the LED. The principle of operation of these prior art devices relies on the fact that a clean groundglass with a specific surface texture exhibits a unique light scattering property in which the scattered light distribution as a function of the angle subtended by the detector surface is critically dependent upon the surface granularity of the groundglass. A clean, oil-free groundglass exhibits the highest light scattering, which results in minimal illumination of the detector and correspondingly low photocurrent. However, when a specimen of oil is applied to the groundglass, the amount of light scattered away from the incident beam direction is reduced so that slightly more light impinges on the detector. This produces a small increase of photocurrent from the detector, and the small difference between the photocurrent readings is the basis for the measurement.
Two major disadvantages and several practical shortcomings are associated with this method. First, the signal level is low, because the major portion of the light emitted from the LED does not fall on the detector. Thus, a very high gain of the signal amplifier is required in order to produce a useful indication. Electronic drift, stray light interference and temperature effects have become a significant source of error in the measurement. Second, the granular surface of the groundglass is difficult to reproduce with consistent properties, which makes the calibration of the measurement system problematic. If plastic is used for the sampling pellet to reduce its cost, contamination of the surface during manufacturing can cause high reject rates, since cleaning method have largely proved ineffective.
It has also been considered to make a plastic "groundglass" by injection molding, in order to produce low cost disposable pellets, which are a prerequisite for widespread application of this concept. However, experience has shown that mold deterioration of injection molding dies is proportional to the surface profile gradients and the number of parts made in the die. A finely textured mold surface is subject to abrasive wear from the recurrent injection of the plastic material, and the optical properties of the molded parts deteriorate early in a production run. The present invention eliminates the above-mentioned disadvantages and practical shortcomings by the use of a smooth optical surface. Such a surface can be molded in large production quantities without mold deterioration. By curving the surface to produce some positive refracting power, the light level at the detector is greatly increased, and the corresponding signal processing is simplified considerably.